1. Field of the Invention
The present invention relates to a method for growing single crystals used for manufacturing single crystals such as those of silicon from which to obtain silicon wafers used as materials for semiconductor devices and, more precisely, to a method for growing single crystals called the MCZ process in which single crystals are pulled by the CZ process from the material melt in a crucible to which magnetic field is applied.
2. Description of the Background
In pulling single crystals using CZ process, as shown in FIGS. 4(a) and 4(b), a quartz crucible 3 is held with a pedestal 4 in a main chamber 1, and a single crystal 6 is pulled with a wire 7 with rotation from the material melt 5 formed in the crucible 3 into a pull chamber 2. Here, oxygen from the quartz crucible 3 dissolves into the material melt 5 in the crucible 3, and as a result oxygen is taken into the single crystal 6. As a larger amount of oxygen in the single crystal 6 will cause various defects in the crystal and deterioration in device properties when processed into semiconductor devices, reduction of oxygen concentration is an important technical problem in drawing single crystals 6 using CZ process. As a technology for solving this problem, there is a process called MCZ jointly utilizing magnetic field application, and processes utilizing cusp magnetic field such as described in Published Patent Application No. 2-1290 are recognized especially effective. This process is the one in which axisymmetrical and radial cusp magnetic field is applied to material melt 5 in a crucible 3 using a pair of circular magnets above and below 8. 8 as jointly shown in FIGS. 4(a) and 4(b). This process is characterized by the way convection of material melt 5 in a crucible 3 is suppressed by allowing the magnetic fields above and below to repulse one another near the material melt 5 to form an axisymmetrical magnetic field bent almost at right angles, thus expanding the field portion lying at right angles across the side wall and the bottom of the crucible.
Material melt 5 is convecting in the crucible 3 along the inside surface as indicated by broken-line arrows. Although fresh material melt 5 is supplied in the vicinity of the inside surface of the crucible 3 due to this convection promoting oxygen elution from the inside surface, a magnetic field lying across the side wall and the bottom of the crucible 3 suppresses convection along the inside wall of the crucible 3, thus suppressing this oxygen elution from the inside surface of the crucible. And in order to enhance this suppression effect it is considered to be necessary to expand the field portion lying at right angles across the side wall and the bottom of the crucible 3, resulting in reduction of field portion lying at right angles across the liquid surface of the material melt 5.
Incidentally, in pulling single crystals by CZ process, the yield is mainly determined by single crystal pulling yield, oxygen yield, and dislocation free yield. The pulling yield is the yield due to deformation of single crystals causing difficulty in continuation of withdrawing, and the oxygen yield is the yield due to change in oxygen concentration distribution in the pulling direction forming areas with oxygen concentration above permissible level. The dislocation free yield is the yield due to first dislocation generation during single crystal pulling causing the pulled region thereafter to become unusable.
And the total yield, namely the actual yield, is determined by the smallest of the three yields, usually being determined by the oxygen yield or the dislocation free yield in CZ pulling using cusp magnetic field. Thus, in CZ pulling using cusp magnetic field, though pulling is typically continued to over 90% of the material melt weight, total yield is actually limited to 60-70% due to first dislocation generation around 60-70% or drastic change in oxygen concentration thereafter. The reason will be explained below.
In pulling single crystals using CZ process, there is a problem of oxygen concentration decrease in the single crystals as pulling proceeds with or without application of magnetic field. Thus, while oxygen in the material melt in the crucible is supplied from inside surface of the crucible, it evaporates as SiO from the free surface of the melt. While the latter evaporation is constant throughout the whole period of pulling, the former supply decreases as time passes because the material melt in the crucible decreases as pulling proceeds reducing the contact area between the two of them. As a result, oxygen concentration in the material melt decreases as pulling proceeds, lowering oxygen concentration in the single crystal.
Here, when cusp magnetic field is applied, while oxygen concentration decreases as convection in the material melt is suppressed lowering oxygen concentration throughout the single crystal, oxygen concentration decrease along the time accompanying the progress of pulling still remains, and in the later period of pulling this oxygen concentration even increases. This phenomenon occurs from various overlapping reasons, one of which is supply of high oxygen concentration material melt from the vicinity of the bottom of the crucible to the interface of the single crystal caused by material melt thickness decrease in the crucible such as described below.
In single crystal pulling using CZ process, as shown in FIG. 4(b), material melt 5 in the crucible 3 is consumed as pulling proceeds, and the surface level of the material melt 5 is lowered. Even though the crucible 3 is gradually raised to prevent this surface level lowering, the melt thickness from the bottom of the crucible 3 to the surface of the melt continues to decrease, and the thickness becomes small in the later period of pulling. Separate from the convection along the inside surface of the crucible 3, a strong upward flow occurs in the central part of the material melt 5 due to relative rotation between the single crystal 6 and the material melt 5, and in the later period when melt thickness becomes small, melt with high oxygen concentration that has stayed near the bottom of the crucible 3 begins to be supplied by this upward flow directly to the interface between the single crystal 6 and the material melt 5. Moreover, in the later period of pulling, as the bottom of the crucible 3 comes close to the center of the cusp magnetic field, the portion of magnetic field lying perpendicularly across the bottom decreases reducing the convection suppression effect to suppress oxygen elution. As a result of these, oxygen concentration increases as time passes even after allowing for the reduction in oxygen elution due to decrease in contact area reduction between the crucible 3 and the material melt 5.
This increase in oxygen concentration becomes a factor determining the yield of single crystal as it is conspicuous in company with the decrease in oxygen concentration accompanying the progress of pulling, though it is confined in the later period of pulling.
On the other hand, recent scaling up in diameter of single crystals relates to the dislocation free yields. Thus, as single crystals become larger, the diameter of the crucible increases accordingly. When temperature gradient of the material melt in the radial direction of the crucible is constant, the crucible temperature increases as the diameter of the crucible becomes larger. The crucible temperature rise will promote elution loss from inside of the quartz crucible causing first dislocation generation of single crystals.
In pulling using cusp magnetic fields, though high values over 90% are secured for yield of pulling, actually the total yield is low because oxygen yield or dislocation free yield is 60%-70%, lower than pulling yield.